This invention relates control of a diesel engine provided with an exhaust gas recirculation device.
Tokkai Hei 8-128361 published by the Japanese Patent Office in 1996 discloses an exhaust gas recirculation (EGR) device which suppresses a combustion temperature of a diesel engine by recirculating part of the exhaust gas to the intake air. This EGR device varies an exhaust gas recirculation amount (EGR amount) according to the opening of an exhaust gas recirculation valve (EGR valve), and when the EGR valve is operated using an actuator, a delay occurs from when a command signal is input to the actuator to when the EGR valve starts operating. In this connection, in the aforesaid prior art, advance processing corresponding to this delay is applied to the command signal of the actuator to enhance the precision of EGR control.
EGR control has characteristics such that, for example, the EGR amount is increased during vehicle deceleration and the EGR amount is decreased during vehicle acceleration. When the diesel engine is running in the transient state, the EGR amount must be varied with good response so as to obtain an appropriate EGR amount according to these control characteristics. In the prior art, the response delay of the actuator is compensated by advance processing, but the time required for the exhaust gas to travel from the EGR valve to the cylinder of the engine was not compensated.
Further, according to the prior art device, the command value of the EGR amount is calculated by a control unit, but as no physical limit is set on the command value obtained, it is possible that a command value which exceeds the maximum EGR amount which can physically be reached by the EGR valve, or a command value below the minimum EGR amount which can physically be reached by the EGR valve will be set, and as a result the response of the EGR control will be impaired.
It is therefore an object of this invention to eliminate the delay of EGR control due to the time required for exhaust gas to flow from the EGR valve to the cylinder of the engine, and to improve the ability of the EGR amount to track the target value.
It is a further object of this invention to prevent the command value of the EGR amount from exceeding the physical limits of the EGR valve.
In order to achieve the above objects, this invention provides a control device for such an engine that comprises a combustion chamber, an intake passage which aspirates air into the combustion chamber, an exhaust passage which discharges exhaust gas from the combustion chamber, and an exhaust gas recirculation valve which recirculates part of the exhaust gas of the exhaust passage into the combustion chamber.
The control device comprises a sensor which detects a running condition of the engine and a microprocessor programmed to set a target exhaust gas recirculation amount based on the running condition, determine a maximum recirculation amount of the exhaust gas recirculation valve;
compare the target exhaust gas recirculation amount with the maximum recirculation amount, limit the target exhaust gas recirculation amount to be equal to the maximum recirculation amount when the target exhaust gas recirculation amount exceeds the maximum recirculation amount, control the opening of the exhaust gas recirculation valve based on the target exhaust gas recirculation amount, and maintain the opening of the exhaust gas recirculation valve at an opening corresponding to the maximum recirculation amount for a predetermined time after the target exhaust gas recirculation amount has fallen below the maximum recirculation amount.
This invention also provides a control device comprising a mechanism for detecting which a running condition of the engine, a mechanism for setting a target exhaust gas recirculation amount based on the running condition, a mechanism for determining a maximum recirculation amount of the exhaust gas recirculation valve, a mechanism for comparing the target exhaust gas recirculation amount with the maximum recirculation amount, a mechanism for limiting the target exhaust gas recirculation amount to be equal to the maximum recirculation amount when the target exhaust gas recirculation amount exceeds the maximum recirculation amount, a mechanism for controlling the opening of the exhaust gas recirculation valve based on the target exhaust gas recirculation amount, and a mechanism for maintaining the opening of the exhaust gas recirculation valve at an opening corresponding to the maximum recirculation amount for a predetermined time after the target exhaust gas recirculation amount has fallen below the maximum recirculation amount.
This invention also provides a control method for the engine comprising, setting a target exhaust gas recirculation amount based on the running condition, determining a maximum recirculation amount of the exhaust gas recirculation valve, comparing the target exhaust gas recirculation amount with the maximm recirculation amount, limiting the target exhaust gas recirculation amount to be equal to the maximum recirculation amount when the target exhaust gas recirculation amount exceeds the maximum recirculation amount, controlling the opening of the exhaust gas recirculation valve based on the target exhaust gas recirculation amount, and maintaining the opening of the exhaust gas recirculation valve at an opening corresponding to the maximum recirculation amount for a predetermined time after the target exhaust gas recirculation amount has fallen below the maximum recirculation amount.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.